Recently, as the design rule of a dynamic random access memory (DRAM) has decreased, a cell region has decreased and an aspect ratio of a storage node in a capacitor has increased greatly. Thus, it has become difficult to secure a dielectric capacity required for each unit cell.
Conventionally, a dielectric film has been formed in oxide/nitride/oxide (ONO) layer structure to secure a dielectric capacity. However, recent researches on forming a dielectric film with an aluminum oxide (Al2O3) layer (∈=9), a hafnium dioxide (HfO2) layer (∈=25), or a stacked layer of HfO2/Al2O3, wherein the Al2O3 layer and the HfO2 layer both have a high dielectric constant, have been actively progressed in attempts to secure a larger dielectric capacity. Also, such dielectric film is formed by employing an atomic layer deposition (ALD) method instead of the conventional chemical vapor deposition (CVD) method in order to respond to the large aspect ratio.
However, for a dielectric film formed with the stacked layer of HfO2/Al2O3, a dielectric characteristic and a leakage current characteristic of the entire dielectric film depend on a corresponding dielectric constant c and a band gap energy Eg of each material. That is, the dielectric film formed with the conventional stacked layer of HfO2/Al2O3 shows an electrical characteristic formed by a combination of the characteristics of each layer, as described hereinafter.
Generally, a dielectric constant ∈ and a band gap energy Eg of the Al2O3 layer are 9 and 9 eV, respectively. On the other hand, a dielectric constant ∈ and a band gap energy Eg of the HfO2 layer are generally known to be 25 and 5.6 eV, respectively. That is, the dielectric characteristic of the entire dielectric film is affected by the HfO2 layer, and the leakage current characteristic is affected by the band gap energy Eg of the Al2O3 layer. On the contrary, the leakage current characteristic of the entire dielectric film is deteriorated due to the low band gap energy Eg of the HfO2 layer, and the dielectric characteristic of the entire dielectric film is deteriorated due to the low dielectric constant of the Al2O3 layer. Thus, if the dielectric film is applied in a capacitor of a DRAM device, a large limitation occurs with respect to decreasing a thickness of the dielectric film.
However, the Al2O3 layer functions to lower a crystallization temperature of the HfO2 layer, which is the other layer consisting the dielectric film, and through such a function, a leakage current of the dielectric film is reduced. Therefore, in order to improve the characteristics of the dielectric film, the dielectric constant and the band gap energy of the oxide material, forming the dielectric film together with the Al2O3 layer, are generally required to be controlled.